As recent IT (information technology) technology is spread, enterprises or individuals create pieces of content that are electronic data and exchange or release the pieces of content on the Internet on a daily basis. Many pieces of content are created as electronic structured documents in a metalanguage format, such as XML (Extensible Markup Language), HTML (HyperText Markup Language), XHTML (Extensible HyperText Markup Language), or SGML (Standard Generalized Markup Language), and processed in various types of computers.
In relation to a structured document, schema data for defining the document structure of the structured document in a schema language format, such as XML Schema, a document type definition (DTD), Regular Language description for XML (RELAX), RELAX Next Generation (RELAX NG), Name space-based Validation Dispatching Language (NVDL), or Schematron, may be created.
Moreover, recently, XML has come to be used as the file format of office applications, and such exemplary XML is Open Document Format (ODF) or Office Open XML (OOXML).